Today, we have commited in the Gallio development trunk, the new syntax for the MbUnit contract verifiers.
That new syntax is a major breaking change. After you update Gallio with the latest version (126.96.36.1999 and later), you will need to modify the declaration of your contract verifiers to make your project compilable again. Download the latest development build here.
Please refer to that post for additional details about the new syntax.